Various filtration devices are known to filter polymers, i.e. molten or dissolved polymers, e.g. prior to spinning filaments from the molten or dissolved polymer using a spin pack or spinnerets.
The filtration devices for polymer filtration usually comprises filter candle, also named candle filters or tubular filters.
A filter candle apparatus for polymer filtration is disclosed in U.S. Pat. No. 5,279,733. The cooperating elements of a filter candle according to U.S. Pat. No. 5,279,733 are shown in FIG. 1 which is a reproduction of FIG. 1 of U.S. Pat. No. 5,279,733.
The filter candle according to prior art comprises a mesh pack 12, which mesh pack is usually provided from a pleated sintered metal fiber medium, creating an axial opening 13. At a first end, an end fitting 16 is fixed in a liquid tight manner to the filter medium and defines an axial opening 18 of the end fitting 16. At the opposite side, the mesh pack is provided with an end cap 14, which is also fixed in a liquid tight manner to the filter medium. In the void space of the mesh pack, defined by the filter medium, the end cap and the end fitting, a core tube 20 is inserted, which core tube 20 has a number of perforations 22 along the wall of the core tube. At one end, the core tube 20 is provided with an end member 24, being a flange and a threaded end 25 for coupling the filter candle to the other parts of the filter apparatus. The core tube is inserted in the axial opening 13 and the opening 18. In order to seal the core tube and the mesh pack in a liquid tight manner, the end fitting is provided with a recess along the opening 18 in which a ring shaped bearing 26 is provided. This ring shaped bearing 26 is a stainless steel spring having a C-shaped cross section, the opening of the C-shape preferably pointing downwards. The mesh pack with inserted core tube is inserted in a basket-like outer guard 28 via an axial opening 30. The guard, mesh pack and core tube are coupled by means of a screw 34. The bearing engages the recess of the end fitting, the end flange and the lower outer side of the core tube. As shown in more detail in FIG. 2, due to the compression provided by the mounting of the mesh pack 12, core tube 20 and guard, the C-shaped ring 26 seals the mesh pack 12 and the core tube 20, prohibiting polymer to flow to the core tube without passing through the filter medium of the mesh pack.
The filter candle, which is to filter polymer “outside-in”, has some disadvantages. Usually, high pressure is used during filtration, e.g. process pressures in the range of 200 bar to 500 bar, and using pressure drops over the filter candle of up to 150 bar, which pressures are necessary due to the high viscosity of the polymer fluid, e.g. about 100 Pa*s to 1000 Pa*s. Due to this pressure, some polymer is forced to flow in the small interstices 41 between the bearing 26 and the mesh pack end fitting 13 or in the small interstices 40 between the core tube 20 and the bearing 26 and end fitting 40. The polymer, which is pressed in the interstices, will no longer flow and harden, causing the polymer to become brittle, usually blackened. After some time, the brittle polymer will loosen and create hard polymer particles in the polymer fluid. The interstices 40 are located at the clean, filtrate side of the filter membrane. So the polymer particles, which are created at these interstices, will flow along with the filtered polymer, and may cause problems in the further process, which uses the filtered polymer. E.g. it may cause spinnerets to be clogged partially, or create black contamination is polymer articles using the filtered polymer.
A second disadvantage of the prior art filter candles as shown in FIG. 1 and FIG. 2, is the reusability of the bearings 26. Once used, the bearings usually have to be thrown away, because they loose part of their springiness, and they can be contaminated by brittle polymer to a too large extent, making cleaning no longer economical or practical.